The invention relates in general to firearms and more specifically to ammunition for firearms.
A significant and longstanding deficiency inherent to ammunition cartridge case designs is that they allow for only a very small amount of case crush during locking of the firearm bolt for a given amount of input force and energy. This deficiency is widespread in both military and commercial applications and affects conventional as well as developmental cartridge designs independent of particular case material.
Headspace is a fundamentally important characteristic to both the firearm and ammunition designer and is the distance from the feature in the cartridge chamber that limits forward movement of the cartridge to the feature in the firearm locking mechanism that limits rearward movement of the cartridge. In the vast majority of firearms, the cartridge chamber is an integral part of the gun barrel, and the bolt face is the feature in the firearm locking mechanism that limits rearward movement of the cartridge. In production firearms, headspace is not a singular value but rather a defined range of acceptable values to allow for component level manufacturing tolerances and/or assembly tolerances. In addition to allowable headspace tolerance, the cartridge case itself also has a manufacturing tolerances to include acceptable deviations in the portion of its length that interacts with the headspace controlling features of the firearm mechanism as previously described.
Prior Art FIG. 1 shows a conventional cartridge with a bottleneck configuration. Prior Art FIG. 2 shows a cross-sectional view of a conventional cartridge with a bottleneck configuration. The cartridge comprises a conventional cartridge case, a primer, an interior volume for propellant and a projectile.
For example, consider a bottleneck cartridge in caliber .308 Winchester, which is a close commercial equivalent to the popular military 7.62×51 mm NATO caliber. Per the Sporting Arms and Ammunition Manufacturers' Institute (SAAMI), recommended chamber headspace and case length limits are defined in publication ANSI/SAAMI Z299.4-2015, which is hereby incorporated by reference. The recommended range of chamber headspace values for this particular caliber are 1.630-1.640 inches. The recommended range of values of the portion of the cartridge case length that interfaces with the chamber headspace controlling features are 1.634-0.007 inches, or 1.627-1.634 inches. If chamber headspace is at its minimum value of 1.630 inches and cartridge case length is at its maximum value of 1.634 inches, there will be an interference condition of 1.630-1.634 or −0.004 inch between the cartridge case head and the bolt face when the cartridge is fully chambered and the bolt is locked. If chamber headspace is at its maximum value of 1.640 inches and cartridge case length is at its minimum value of 1.627 inches, there will be a clearance condition of 1.640-1.627 or 0.013 inch between the cartridge case head and the bolt face when the cartridge is fully chambered and the bolt is locked.
In the minimum chamber and maximum case scenario, the resulting interference in the axial direction is referred to as case crush. As its name implies and in order to fully lock, the firearm locking mechanism must deform/crush the chambered cartridge case by an amount equal to the interference. In terms of practical implementation, ease of use, and maintaining reliable function, the maximum amount of case crush imposed by the firearm designer, by way of chamber headspace definition, is limited by the required amount of input force and energy to crush the case. For manually operated firearms (bolt action rifle, for example), if too much case crush were imposed, the operator may not be able to lock the bolt as the force required to do so may exceed what is achievable by a person of typical stature and strength. For self-powered, auto cycling firearms (open bolt fired machine gun, for example), excessive case crush demands may require an amount of energy that exceeds the percentage of firearm operating group counter recoil energy available for this specific event resulting in either the locking mechanism being unable to fully lock or, if able to fully lock, reducing the firing pin impact velocity and/or impact energy to the point of inducing cartridge misfires.
In the maximum chamber and minimum case scenario, clearance in the axial direction exists between the locked bolt face and chambered cartridge case head. The amount of possible clearance is effectively limited by the material properties of the cartridge case and its ability to accommodate deformation without structural compromise or failure. From a producibility perspective in terms of reducing manufacturing costs, it is preferable to impose chamber headspace limits that allow for the maximum amount of clearance as this translates into larger tolerances for the manufacture and assembly of the firearm components that contribute to chamber headspace. The downsides to increasing clearance, however, are in accepting a decreased level of position control of the chambered cartridge as well additional structural demands on the bolt locking features due to the impulsive impact load applied by the case head to the bolt face during firing. Decreased position control of the chambered cartridge leads to inconsistencies in the initial launch angle of the bullet as it departs the case, which subsequently contributes to degraded downrange accuracy and precision. As for the impact loading of the case head onto the bolt face during firing, this phenomena is often addressed by the firearm designer by applying a load/scale factor to the combined stress calculations governing the bolt features that structurally support the firing event. An impact load factor is applied to the pressure induced stresses in order to ensure survivability and/or acceptable fatigue life. If the firing forces were not of an impactful nature, bolt life would immediately increase (without any design changes), or the bolt could be redesigned to a smaller/lighter version (while maintaining same life expectations of existing bolt).
In summary, cartridge case designs inherently only allow for a very small amount of case crush to take place during locking of the firearm bolt, which subsequently leads to allowable clearance between the bolt face and cartridge case head when firing production ammunition. This has three unique and significant consequences. The first is degraded firing accuracy and precision. The second is that increased structural demands are placed on the locking features of the bolt, which leads to decreased life or larger/heavier designs. The third is that it limits the allowable manufacturing and assembly tolerances for components influencing chamber headspace, which increases cost.
A need exists for a cartridge case which allows for a more substantial amount of case crush to take place during the locking of the firearm bolt.